The present invention relates generally to methods of and devices for treating a region of a subject with a radiotherapy beam and more particularly to such a method and device wherein the region is irradiated by the beam substantially simultaneously with a magnetic resonance imaging system imaging the region in such a manner that the beam is not incident on a coil assembly of the imaging system.
Radiotherapy machines, such as the CLINAC machines manufactured by the assignee of the present invention, generally include a linear electron beam accelerator mounted on a gantry which rotates on an approximately horizontal axis. The electron beam accelerator is usually mounted on the gantry in such a manner that it is offset from the horizontal rotational axis of the gantry. The high energy electron beam emerging from the accelerator is further processed by techniques well-known to those experienced in the art to produce either an electron beam or an X-ray beam suitable for patient treatment. In either case the radiation is collimated in a treatment beam which is caused to travel in a direction perpendicular to the rotational axis of the gantry in such a manner that the axis of the treatment beam intersects the rotational axis of the gantry. The point at which the axis of the treatment beam intersects the rotational axis of the gantry is the focal point of the treatment beam and is referred to as the isocenter of the radiotherapy machine.
In a radiotherapy machine the patient is placed on a treatment couch that can be precisely positioned to locate the treatment region, which is usually a cancerous tumor or lesion in the patient, on the rotational axis of the gantry at the isocenter of the radiotherapy machine. Thus, by rotating the gantry, the source of the treatment beam can be rotated around the patient during treatment, thereby minimizing the amount of treatment radiation passing through any one region of the patient""s body near the treatment region while the beam always passes through the treatment region itself. Excessive irradiation of non-diseased tissue, especially those tissues abutting the diseased treatment region, causes undesirable cell damage and cell death in healthy tissue.
Among practitioners of current radiotherapy treatment art it is well-known that minimum abutting cell damage generally occurs when the diseased treatment region in the patient is precisely located at the isocenter of the radiotherapy machine. However, several limitations of the present art make it difficult to achieve the desired precise positioning of the diseased region of the patient at the isocenter of the radiotherapy machine.
One reason for this difficulty is that diseased tissue in a patient usually is surrounded by, or is adjacent to, other soft tissue which is materially similar to the diseased tissue. The similarity of the tissues makes it difficult to precisely define the exact boundaries of the diseased tissue using current diagnostic and imaging techniques appropriate for radiotherapy machines.
One past attempt to overcome this problem has involved using relatively low contrast two-dimensional X-ray-based imaging of the region when the subject is positioned on the radiotherapy machine. The X-ray-based imaging systems have generally relied on detecting X-rays in the same X-ray beam which is used for radiotherapy purposes. However, low contrast two-dimensional X-ray-based imaging of the region does not enable the true position of the region including the tumor or lesion to be definitely located. The difference in X-ray absorbance between different soft tissue structures and between cancerous and non-cancerous soft tissues frequently ranges from small to undetectable. Only the bones, which absorb X-rays more strongly, can be readily imaged and precisely located by this means. Determining the true position of the soft tissue region to be treated is difficult because due to its lack of rigidity the region moves relative to the nearby bones of the subject as a result of unavoidable body movements of the subject on the treatment couch. The uncertainty in determining the true position of the region exists even when fiducial markers are inserted into the tumor because patient movement is likely to cause the fiducial markers to move.
Because the region desired to be treated is usually not located exactly as planned with respect to the isocenter of the radiotherapy system, insufficient quantities of radiotherapy beam energy are deposited in the region desired to be treated and excessive amounts of radiotherapy beam energy are deposited in healthy tissue in a volume abutting the region desired to be treated. Consequently, the tissue in the abutting volume is subjected to undesired and unnecessary damage so healthy organs adjacent the tumor site are damaged.
Because of the general inability to focus the radiotherapy beam with sufficient precision on the region desired to be treated, current medical practice is to increase the irradiated area to include additional tissue volume and to increase the dosage of the radiotherapy beam to ensure complete cell death in the region desired to be treated. The expectation is that all cells in the treated region are killed and possible positioning errors between the beam and the region are compensated. However, such techniques inevitably cause increased collateral radiation damage to the volume abutting the desired region to be treated, in some cases resulting in devastating quality of life effects on the subject. It is, accordingly, an object of the present invention to provide a new and improved method of, and apparatus for enabling a radiotherapy beam to be accurately positioned on a desired region to be treated by the beam.
Another object of the invention is to provide a new and improved method of, and apparatus for enabling a radiotherapy beam to be precisely positioned on a region desired to be treated, wherein the apparatus used to determine whether the beam is properly located is easily retrofitted on existing radiotherapy devices.
An additional object of the invention is to provide a radiotherapy machine including a magnetic resonance imaging system for acquiring 2D and 3D spatially resolved high-contrast images of soft tissue structures and organs within and abutting the region desired to be treated.
An additional object of the invention is to provide a radiotherapy machine including a magnetic resonance imaging system, wherein an excitation coil assembly of the imaging system is arranged so that a radiotherapy beam of the radiotherapy machine is not incident on the coil assembly and wherein the coil assembly is arranged so subjects to be treated can easily be placed in the path of the radiotherapy beam, on a treatment couch.
An additional object of the invention is to provide a new and improved radiotherapy machine in combination with a system for directly detecting the effect of the radiotherapy beam on an irradiated region, particularly the contents of tissue cells in the region, and to spatially resolve the effect of the irradiation to enable real time three-dimensional correlation between the shape, position and intensity of the region actually being irradiated and the known location of a region desired to be irradiated, where a tumor or lesion is located.
A further object of the invention is to provide a new and improved radiotherapy machine in combination with a relatively low cost device for determining whether, and the degree to which tissue in a region desired to be treated by a radiotherapy beam is actually being treated.
Still a further object of the invention is to provide a new and improved X-ray beam therapy device in combination with a magnetic resonance imaging system, wherein secondary electron skin dosage resulting from bombardment of the skin by the X-ray beam is substantially reduced by the magnetic field of the coils of the imaging system.
In accordance with one aspect of the present invention, these and other objects are achieved by treating a region of a subject with a radiotherapy beam while the region and volumes abutting the region are imaged by a magnetic resonance imaging system. The beam and an excitation coil assembly of the imaging system are arranged so the beam is not incident on the coil assembly and also so that magnetic fields derived from the coil assembly do not perturb the particle trajectories in the beam in the case where the radiotherapy beam is composed of charged particles such as electrons.
The excitation coil assembly of the imaging system preferably includes first and second spaced segments for producing a main DC magnetic field; the segments are located on opposite sides of the region.
In one embodiment, wherein the excitation coil assembly is mounted independently of movement of the treatment beam axis, the first and second excitation coil assembly segments have a common axis substantially coincident with an axis that passes through the region to be treated and about which the beam axis turns. A subject carrying structure, e.g., a treatment couch, fits within aligned central openings of the coil segments. The beam axis passes between the two segments at right angles to the main magnetic field lines produced by and extending between the segments.
In other embodiments, the coil assembly is mounted so it moves as the beam axis moves.
In one of these embodiments, each of the first and second coil segments includes a central opening having a common axis. The beam axis extends through the central openings of both coil segments and is generally aligned with magnetic field lines established by and extending between these segments. In another embodiment, the beam axis extends through a space between the segments and is generally at right angles to magnetic field lines established by and extending between the segments. The latter arrangement, which does not have a central opening in the coil segments, is advantageous because it establishes a higher intensity magnetic field than the arrangements with such an opening.
A feature of the invention is that the magnetic field derived from the excitation coils of the magnetic resonance imaging system is relatively low, sufficient to provide only the minimum necessary spatial resolution and sensitivity for determining whether the radiotherapy beam is incident on the desired region to be treated. The magnetic field density is sufficiently low that conventional copper-wound water cooled coils can be employed to generate the main magnetic field, although superconducting magnetic coil assemblies, cooled to a liquid helium or liquid nitrogen temperature, can be employed if desired.
If a liquid helium-cooled superconducting coil generates the magnetic resonance imaging system main magnetic field, commercially available high temperature superconducting supply leads preferably establish external connections between the superconducting coil and a DC power supply for exciting the superconducting coil. The high temperature superconducting supply leads block heat leakage from the liquid nitrogen temperature at which they are maintained, i.e., 77xc2x0 K., to the low temperature superconducting coil at 4.2xc2x0 K. Thereby, the low temperature superconducting coil is not required for reasons of heat leakage to operate in the persistent mode without supply leads connected, hence the current in the coil can be pulsed on and off by an external supply without an unacceptable increase in liquid helium consumption. By pulsing the high temperature superconductor supply leads on and off synchronously with pulsing an electron radiotherapy beam off and on, the magnetic resonance imaging system has no adverse deflection effects on electrons in the radiotherapy electron beam.
To reduce the required strength of the magnetic field derived from the resonance imaging system magnetic coils, a radio frequency pickup coil of the resonance imaging system is preferably a superconductor. This enables the main magnetic coil of the resonance imaging system to have a relatively small size, to facilitate retrofitting the main coil to existing radiotherapy machines and reduce the cost of a facility including the structure of the invention. The superconducting radio frequency coil is preferably a high temperature superconductor formed from oriented high temperature superconducting films grown on metal foils or on planar oxide single crystal substrates. A further feature of the invention is that leakage magnetic fields originating in the radiotherapy machine are decoupled from magnetic fields originating in the magnetic resonance imaging system and leakage magnetic fields originating in the magnetic resonance imaging system are decoupled from the radiotherapy linear accelerator. The decoupling is preferably provided by compensating coils positioned outside the imaging system coils and by a coil or coils surrounding the linear accelerator and its associated components.
A feature of the present invention is the capability of the magnetic resonance imaging system to detect changes in nuclear magnetic resonance spectral parameters of the image region due to the effects of the radiotherapy beam irradiating the tissue desired to be treated by the beam.
At thermal equilibrium in a magnetic field, the magnetic moment of a nucleus is aligned with the magnetic field. When perturbed from this alignment, the magnetic moment precesses around the applied field at the characteristic resonance frequency of the particular nuclear species (often a hydrogen nucleus or proton). The variation of the applied magnetic field at different atomic sites in a molecule due to the shielding effects of the surrounding electrons causes small shifts in the resonance frequencies of similar nuclei. The nature of the environment of the resonating nuclei determine the rate at which the resonance decays, or relaxes. These differences in resonance frequencies can be resolved and used to analyze molecular structures. Alternatively, the resonance frequencies may be modified by the imposition of a magnetic field gradient across a sample in which case the resonance frequency is a function of the position of a particular nuclear spin within the sample. This forms the basis of nuclear magnetic resonance imaging systems.
In either case, application of an rf pulse with a frequency close to that of the natural resonance frequency of the spins is used to perturb the nuclear magnetic moment. The perturbation rotates the nuclear magnetic moment away from its alignment with the applied magnetic field. A rotation of 90xc2x0 produces a maximum magnetization transverse to the magnetic field while a 180xc2x0 rotation results in an inversion of the initial magnetization but no transverse magnetization. It is the transverse component of magnetization which precesses about the applied magnetic field and which can be detected in an NMR spectrometer. Following a perturbation, two relaxation times characterize the return to thermal equilibrium. In addition to precessing, the transverse magnetization decreases in amplitude with a characteristic time constant T2, the spin-spin relaxation time. The component of magnetization parallel to the applied field returns to its initial value with a characteristic time constant T1, the spin-lattice relaxation time. Both of these relaxation times are affected by the magnetic influences of neighboring atoms and molecules. In particular, the presence of free radicals with their strong electronic magnetic moment can modify the relaxation times and resonance frequencies of nearby nuclei. The measurement of resonance frequencies and relaxation times may be combined with NMR imaging methods to provide NMR spectral data which is correlated with spatial position.
Because of the high sensitivity of the NMR spectral parameters to differences in the magnetic environment of nuclei in different types of soft tissue i.e. between the tissues in different body organs or between cancerous and non-cancerous tissues, the NMR image of soft tissue structures achieves much greater contrast than an X-ray image of the same tissue volume. Additionally the MR image contains 3D rather than 2D positional information. The position of the cancerous tumor or lesion in the soft tissue can therefore be determined directly on the radiotherapy machine with much greater precision than by inferring the presumed position of the tumor with reference to the position of the nearby bones obtained from an X-ray image.
In addition, because of the large change in NMR spectral parameters induced by the presence of free radicals, which are one of the primary products of the irradiation of tissues by the radiotherapy beam, both the spatial location and the intensity of the irradiation effects of the radiation therapy beam on the tissues within the imaged volume can be determined in real time during treatment.
In accordance with an aspect of the present invention, the analytical capability of a magnetic resonance imaging system is used to detect changes in nuclear magnetic resonance spectral parameters due to effects of the radiotherapy beam irradiating the tissue. The radiotherapy beam incident on selected tissue causes free radicals and ionization products to be produced in the tissue. The presence of these can be detected and imaged. The radiotherapy beam, the high intensity magnetic fields and the rf pulse of the magnetic resonance imaging system thus interact to enable three-dimensional spatial distribution information to be derived for the radiation dose of the radiotherapy beam deposited in the treated and abutting tissue during the treatment process. The three-dimensional information is derived by using known magnetic resonance imaging techniques and by correlating the detected data with the beam axis position and the known beam cross-sectional geometry and intensity. The three-dimensional information relating to the spatial distribution of the radiotherapy beam on the treated tissue is correlated with previously gathered and therefore known three-dimensional data concerning the position of the cancerous tissue desired to be treated. Thereby, the radiotherapy beam can be confined to the tissue desired to be treated and controlled so it is not incident on the abutting tissue. This enables the total radiation dose to the subject from the beam to be reduced and collateral damage to healthy tissue minimized. An MR image in absence of radiation will show cancerous tissue. With the beam on, the MR image will show the extent of tissue being irradiated.
The above and still further objects, features and advantages of the present invention will become apparent upon consideration of the following detailed description of specific embodiments thereof, especially when taken in conjunction with the accompanying drawings.